Several composite products comprising a metal matrix embedding a strengthening or reinforcing phase comprising a filler such as ceramic particulates, whiskers, fibers or the like, show great promise for a variety of applications because they combine the strength and hardness of the strengthening phase with the ductility and toughness of the metal matrix. Generally, a metal matrix composite body will show an improvement in such properties as strength, stiffness, contact wear resistance, and strength retention at elevated temperatures relative to the matrix metal, per se. In some instances, the composite bodies may be lighter in weight than correspondingly sized bodies of the matrix metal per se. However, the degree to which any given property may be improved depends largely on the specific constituents used, their respective volumes or weight fractions in the composite bodies and how they are processed in forming the composite bodies. Aluminum matrix composites reinforced with ceramic fillers such as silicon carbide in particulate, platelet or whisker form, for example, are of interest because of their higher stiffness, and greater wear and temperature resistance relative to unfilled aluminum.
Various metallurgical processes have been described for the fabrication of aluminum matrix composites, including methods based on powder metallurgy techniques and those based on molten metal infiltration of reinforcing materials, such as by pressure casting.
With powder metallurgy techniques, the metal in the form of a powder and the ceramic reinforcing material in the form of a powder, whiskers, chopped fibers, etc., are admixed and then either cold-pressed and sintered, or hot-pressed. The production of metal matrix composites by powder metallurgy utilizing conventional processes imposes certain limitations with respect to the characteristics of the products attainable. The volume fraction of the ceramic phase in the composite is limited, typically to about 40%, the pressing operation poses a limit on the practical size attainable, and only relatively simple product shapes are possible without subsequent processing (e.g., forming or machining) or without resorting to complex presses. Also, nonuniform shrinkage during sintering can occur, as well as nonuniformity of microstructure due to segregation in the compacts and grain growth.
When molten aluminum is employed in the fabrication of, for example, aluminum matrix-alumina filled composites, the molten aluminum does not readily wet alumina reinforcing materials, thereby making it difficult to form a coherent product. The prior art suggests various solutions to this problem including coating the alumina (or other filler materials) with a wetting agent, applying pressure to force the molten aluminum into the reinforcing material or filler, applying a vacuum to draw the molten aluminum into the filler, operating at very high temperatures, well above the melting point of aluminum, and a combination of these techniques. These techniques tend to complicate the processing, require expensive equipment such as presses, vacuum apparatus, controls, etc., limit the sizes and shapes of products which can be formed, and sometimes introduce undesirable components into the product in the form of wetting agents or the like.
The use of a reactive atmosphere entrapped in a mold to facilitate the infiltration of molten metal is disclosed by U.S. Pat. No. 3,364,976 to J. N. Reding, et al. This patent discloses a method of casting metals such as aluminum and magnesium alloys in which a mold cavity, optionally containing a suitable filler, contains an atmosphere which is reactive with the molten metal to be cast and forms a low volume, solid reaction product. The mold is effectively sealed so that the reaction with the molten metal consumes the entrapped atmosphere and generates a vacuum within the mold cavity, thereby drawing in the molten metal. For example, at col. 3, line 55 et seq., there is described the reaction of molten magnesium with the oxygen and nitrogen content of the air to form magnesium oxide and magnesium nitride, thereby generating a vacuum sufficient to substantially completely fill the mold with molten magnesium. The drawings illustrate a box-like mold 10 having a single opening 12 leading to a cavity 14 containing an atmosphere which is appropriately reactive with molten metal 16. Immersion of the mold into a body of the molten metal, as illustrated in FIG. 3, is stated to obviate the necessity for the mold to be entirely gas or liquid tight (col. 2, lines 57-61) and reaction of the atmosphere entrapped within the mold causes the molten metal to fill the mold. Examples 5 and 10, respectively, illustrate infiltration of an alumina grain with molten magnesium alloy at 1300.degree. F. (704.degree. C.) and infiltration of a silicon carbide with molten aluminum alloy containing 5% magnesium at 1400.degree. F. (760.degree. C.).
U.S. patent application Ser. No. 049,171, filed May 13, 1987, which issued on May 9, 1989, as U.S. Pat. No. 4,828,008, in the name of Danny R. White, et al. and entitled "Metal Matrix Composites", assigned to the assignee of this application, discloses a method for producing aluminum matrix composites. According to this method, molten aluminum containing at least about 1 weight percent magnesium, and preferably at least about 3 weight percent magnesium, is contacted with a permeable mass of ceramic filler in the presence of a gas comprising from about 10 to 100 volume percent nitrogen, balance nonoxidizing gas, e.g. argon or hydrogen. The molten aluminum alloy, which may be at a temperature of about 700.degree. C. to 1200.degree. C., spontaneously infiltrates the permeable filler, i.e. infiltrates the filler without the necessity of applying mechanical pressure or vacuum to aid the infiltration. The molten body is allowed to solidify to form a metal matrix body embedding the ceramic filler, i.e. a metal matrix composite body. The ceramic fillers include, for example, oxides, carbides, borides and nitrides, e.g. alumina.